Kibana versions before 6.6.1 contain an arbitrary code execution flaw in the security audit logger. If a Kibana instance has the setting xpack.security.audit.enabled set to true, an attacker could send a request that will attempt to execute javascript code. This could possibly lead to an attacker executing arbitrary commands with permissions of the Kibana process on the host system.
https://www.elastic.co/community/security
https://discuss.elastic.co/t/elastic-stack-6-6-1-and-5-6-15-security-update/169077